1. Field of the Invention
The present invention relates to a piezoelectric driving type MEMS apparatus that is manufactured utilizing a MEMS (Micro-Electro-Mechanical Systems) technique.
2. Related Art
In recent years, attention is paid to a technique for manufacturing such a high frequency element as a variable capacitor or a switch utilizing a MEMS. A variable capacitor obtained by the MEMS has such an advantage that a Q value thereof is higher than that of a variable capacitance diode. On the other hand, the MEMS switch has such merits that an insertion loss thereof is low and isolation property thereof is excellent compared to PIN diode and GaAsFET based-switch (for example, see U.S. Pat. No. 4,670,682). The merits come from a feature of the MEMS that can manufacture a mechanically movable portion.
In order to manufacture the mechanically movable portion, it is necessary to provide an actuator for converting an electrical signal to a mechanical behavior. Actuators can be classified to some types according to their driving systems. As well-known driving systems, there are ones of an electrostatic type, a thermal type, an electromagnetic type and a piezoelectric type. The piezoelectric type driving system is constituted to realize a movable structure utilizing a piezoelectric effect of piezoelectric material. The piezoelectric type actuator has such an advantage that both a low voltage operation and a low power consumption can be realized. Therefore, an MEMS variable capacitor or a MEMS switch utilizing a piezoelectric type actuator is suitable for a high frequency part for a portable device or equipment.
A conventional MEMS variable capacitor employs such a structure that a lower electrode for the variable capacitor is provided at a central portion of a substrate, supporting portions are provided at both ends of the substrate, and a beam which is supported by the supporting portions to displace toward the substrate is provided. The beam is provided with a first insulating film, a first electrode film that is provided on the first insulating film to extend from one end of the beam to the other end thereof, piezoelectric films which are provided on both end portions of the first electrode film except for a central portion thereof, second electrode films which are provided on the piezoelectric films, and a second insulating film which covers the first and second electrode films. As material for the piezoelectric film, PZT, AlN, ZnO, or the like is used. Incidentally, the first electrode film serves as an upper electrode for the variable capacitor.
When different voltages, V1 and V2, are respectively applied to the first electrode film and the second electrode film the piezoelectric films strain so that the length of the beam in its extending direction (hereinafter, “X-axis direction”) varies. When it is assumed that a length Lx of the piezoelectric film in the X-axis direction has changed to Lx+ΔLx due to voltage application, a strain εx=ΔLx/Lx can be expressed by the following equation (1).εx=d31(V1−V2)/t  (1)Here, t represents a thickness of a piezoelectric film, and d31 represents a piezoelectric constant. The piezoelectric constant d31 is a parameter which represents amounts of strain occurring in the X-axis direction and in a direction (hereinafter, “Y-axis direction) orthogonal to the X and Z axes and a film thickness direction of the piezoelectric film (hereinafter, “Z-axis direction”) when electric field is applied in the Z-axis direction, whose value varies according to piezoelectric material. The beam including the piezoelectric films flexes in the direction of the substrate due to strain in the piezoelectric film so that a distance between the first electrode (film) and the lower electrode changes. A change δz of the distance between the electrodes meets the following relationship or equation (2).δz∝ d31(V1−V2)Lx2  (2)
Accordingly, according to increase of a length of the piezoelectric film in the X-axis direction, namely, a length of the beam, a variable range of the capacitor is increased.
Since a cavity is formed under the upper electrode in an MEMS variable capacitor with such a structure, there is such a drawback that, when an acceleration is applied to the MEMS variable capacitor, the upper electrode may move, which results in change in capacitance value. In order to make it harder for the upper electrode to move even when acceleration is applied to the MEMS variable capacitor, such a constitution can be employed that the beam and the upper electrode are reduced in weight and a width Ly of the beam which supports the upper electrode is increased. When the MEMS variable capacitor is mounted to a portable device, there is a high possibility that the portable device is used under an environment where acceleration is applied to the portable device. Therefore, such a countermeasure as widening of the beam becomes important among others.
However, when the width Ly of the beams is increased, the piezoelectric film also strains in the Y-axis direction at a time of application of voltage to the first and second electrodes. A strain εy (=ΔLy/Ly) in the Y-axis direction can be expressed as follows:εy=d32(V1−V2)/t  (3)Here, d32 represents a piezoelectric constant. The beam flexes in the Y-axis direction toward the substrate due to the strain. As a result, such a problem occurs that the upper electrode and the lower electrode do not become parallel to each other so that a desired capacitance value can not be obtained. Incidentally, a displacement amount due to flexion, namely, δy is proportional to square of the beam width Ly.
The flexion of the beam also causes a problem in a piezoelectric type MEMS switch. In order to prevent the isolation property during turning-off of the MEMS switch from depending on acceleration, it is necessary to increase the width of the beam in the MEMS switch. As a result, however, the flexion of the beam also occurs in the Y-axis direction during voltage application. Therefore, when the switch turns on, the electrodes at a contact portion do not become parallel to each other, and they come in contact with each other at only one point. As a result, a resistance occurring when the switch turns on increases and an insertion loss increases so that a desired property can not be obtained. Further, the increase in resistance tends to cause malfunction in the switch due to melting of the electrodes at the contacting portion.